This invention relates generally to injection molding and, more particularly, to a new valve gated melt transfer system for stack molding.
Stack molding provides the advantage that it enables the machine output to be doubled without increasing its size. The location of the cavities on opposing faces of the moving center section or platen makes this possible because the same machine forces are applied to two cavities in series rather than to a single cavity or parting line. However, stack molding has the disadvantage that the melt runner system must extend through the moving center platen to reach the cavities.
In the past, the melt has been conveyed across the first parting line between the stationary platen adjacent the molding machine and the moving platen by a heated sprue tube or bar through the stationary platen from the machine nozzle across to the moving center platen. This structure has the disadvantages that the heated tube is is the way when the mold is open and damages falling parts. Insulating it takes space and thereby reduces the number of cavities possible on the first parting line. Furthermore, when the heated tube loses contact with the machine nozzle, melt drools out and provides undesirable air entry into the system. In addition, the melt conveying passage is longer because it has to cross the opening when the mold is open, thus resulting in higher pressure drop.